ipv6: tcp: fix panic in SYN processing

commit 72a3effa

 ([NET]: Size listen hash tables using backlog
hint) added a bug allowing inet6_synq_hash() to return an out of bound
array index, because of u16 overflow.

Bug can happen if system admins set net.core.somaxconn &
net.ipv4.tcp_max_syn_backlog sysctls to values greater than 65536

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>